When a high molecular weight thermoplastic resin is extruded through a die, smooth extrudates can only be obtained up to a certain shear stress (i.e., shear rate; extruder output rate). Beyond that, surface irregularities begin to appear. Irregularities such as haze and surface roughness, known in the art as melt fracture or "sharkskin," limit the production rates in commercial applications. To counteract these undesirable phenomena, and thereby achieve higher output rates, process aids are typically added to the thermoplastic resin prior to extrusion. One of the primary functions of a process aid is to delay, suppress, or eliminate, the onset of surface flow defects such as haze and sharkskin and to enable operation at higher shear stress.
For example, certain fluoroelastomers have been found to delay the onset of melt fracture and the above mentioned surface defects in thermoplastics such that higher shear rates and output can be attained while still producing acceptable extrudates. Such additives are typically employed at a level of about 250 to 3,000 parts per million based on the weight of the thermoplastic. They are generally added to the thermoplastic resin prior to extrusion by dry blending a fluoroelastomer concentrate therewith. These fluorine-containing elastomers are, however, quite expensive and other systems have been proposed.
Process aids for thermoplastic polymers which preferably consist of a combination of a silicone-glycol copolymer and a phosphorous adjuvant were disclosed by Leung et al. in U.S. Pat. No. 4,925,890. The silicone-glycol actually illustrated as a process aid for various polyolefins and a polycarbonate thermoplastics is a polydimethylsiloxane having carbinol-terminated poly(ethylene oxide) grafts (i.e., --COH end group on the side chains). This patent makes no distinction with respect to the materials of construction of equipment used to process these compositions, mild steel and chrome-plated extrusion die inserts being the only types illustrated.
A similar extrusion method is disclosed in EP Publication 0722981 to Dow Corning Corp., published Jul. 24, 1996. In this case, 0.01 to 1 part by weight of a hydroxy-functional diorganopolysiloxane (i.e., a silanol or --SiOH functional polymer) is added to 100 parts by weight of a polyolefin resin. The resulting composition can be extruded at relatively high rates with little sharkskin formation. Again, there is no suggestion that die construction is critical and only a P-20 alloy steel (i.e., a non-nickel containing steel) die was employed in the examples.